Secondary recovery of oil from a subterranean formation



United States Patent 3,262,498 SECONDARY RECOVERY OF OIL FROM ASUBTERRANEAN FORMATION Carl Connally, Jr., and Elliott B. Elfrink,Dallas, and Lorld G. Sharp, Irving, Tex., assignors to Mobil OilCorporation, a corporation of New York No Drawing. Filed June 24, 1963,Ser. No. 290,167 5 Claims. (Cl. 1669) This invention relates generallyto the recovery of oil from subterranean reservoirs. More specifically,this invention is directed to a method of recovering oil by a misciblephase displacement technique wherein the flood efficiency is improved bythe use of carbon dioxide.

When a well is completed in a subterranean reservoir, the oil present inthe reservoir is normally removed through the well by primary recoverymethods. These methods include utilizing native reservoir energy in theform of water or gas existing under sufiicient pressure to drive the oilfrom the reservoir through the well to the earths surface. This nativereservoir energy most often is depleted long before all of the oilpresent in the reservoir has been removed from it. Additional oilremoval has been effected by secondary recovery methods of adding energyfrom outside sources to the reservoir either before or subsequent to thedepletion of the native reservoir energy.

Miscible phase displacement techniques comprise a form of enhancedrecovery in which there is introduced into the reservoir through aninjection well a fluid or fluids which are miscible with the reservoiroil and serve to displace the oil from the pores of the reservoir anddrive it to a production well. The miscible fluid is introduced into theinjection Well at a sufficiently high pressure that the body of fluidmay be driven through the reservoir where it collects and drives thereservoir oil to the production well. The present invention isparticularly concerned with a miscible slug type of miscible phasedisplacement.

In one form of the miscible slug method, a liquefied hydrocarbon slug isdeveloped within the reservoir by introducing through the injection wella condensable hydrocarbon, such as liquefied petroleum gas, propane, orbutane, at such a pressure that the hydrocarbon will be reduced to theliquid phase or will remain in the liquid phase. Also, a normally liquidhydrocarbon, such as light naphtha, may be introduced into theformation. The liquefied hydrocarbon slug is miscible with the reservoiroil and is driven through the formation to recover the oil. Othermiscible materials may be employed, such as a diluted hydrocarbon slugcomprising the above-identified condensable hydrocarbons diluted withnatural gas. The diluted slug may be a liquid or it may be diluted tothe extent that it is an enriched gas which will be miscible with thereservoir oil. In carrying out the miscible slug technique, a drivinggas is normally injected into the formation behind the hydrocarbon fluidslug in order to drive the slug through the reservoir formation to theproduction well. Generally, the miscible slug technique is carried outat pressures of about 1000 pounds per square inch gauge and higher.

In carrying out the miscible slug technique, it has been found thatserious problems sometimes develop with respect to maintaining a uniformflood front as the liquid progresses through the formation toward theproduction 3252,48 Patented July 26, 1966 ice well. The uniformity towhich the flood pattern, that is, the pattern assumed by the body ofdisplacing liquid, may be held is generally referred to as the sweepefficiency of the flood. When the flood breaks away from a patternhaving a uniform front boundary, generally a portion or portions of theflood will prematurely advance to the production well, resulting in theleaving behind of substantial quantities of reservoir oil. Thisunevenness of flood boundary is often referred to as fingering. Thesweep efiiciency of a flood pattern is considered from the standpoint ofboth the horizontal and the vertical. The horizontal pattern of a flood,that is, the configuration of the flood pattern in a horizontal planeextending through the formation perpendicular to the injection andproduction wells, is generally referred to as the areal sweep. The floodpattern along a perpendicular plane extending through the formationbetween the injection and production wells is referred to as thevertical sweep. The horizontal and vertical sweep efliciencies of aflood pattern are influenced by several factors including the mobilityratio of the displacing to the displaced fluid, which in essence is ameasurement of the relative ease with which the fluids move through theformation. If a fiuid which is not very mobile is being displaced by afluid which is very mobile, the result is a very inefficient sweeppattern in which rapid fingering develops, with the very mobile fluidadvancing in finger-shaped extensions into and ahead of the fluid whichis being displaced. For example, if a low viscosity gas is being used todisplace a viscous oil, obviously the gas will develop into a number offingerlike patterns which will rapidly advance through the body of oiland ultimately reach the production well leaving behind substantialportions of the oil. The mobility of reservoir oil and the fluidsemployed to displace the oil is directly affected by the viscosity ofthese materials.

Thus, it would seem that if the viscosities of displaced and disp lacingfluids canbe altered to the extent that dif# ferences of any magnitudealong any particular line or zone in a flood will be minimized, theflood pattern will be improved.

It is one object of the present invention to provide an improved methodfor the recovery of oil from a subterranean reservoir. It is anotherobject of the invention to provide an improved miscible phasedisplacement technique for recovering oil from a subterranean reservoir.It is a still further object of the invention to provide a miscibleliquid hydrocarbon slug type of miscible phase displacement wherein thesweep efiiciencies of the method are improved.

In accordance with the invention, reservoir oil is recovered from aformation through a production well by injecting into the formationthrough an injection Well a quantityof carbon dioxide which is displacedby a liquid hydrocarbon slug driven by a fluid displacing material.

The first step in the method of the invention is the introduction ofcarbon dioxide through an injection well into the formation from whichoil is to be recovered. The carbon dioxide preferably is introduced intothe injection well in the liquefied state because less energy isrequired than when handling it in the gaseous state. As the liquidcarbon dioxide descends in the wellbore, it undergoes a naturallyincreasing temperature, causing it to become gaseous either in thewellbore or in the formation in the immediate vicinity of the wellbore.Since the temperature of most reservoirs will be above the criticaltemperature of carbon dioxide, 87.8 F., the liquid carbon dioxide will,in most instances, quickly pass from the liquefied to the gaseous stateupon rejection. While the injection pressure for the carbon dioxide isnot particularly critical, it is preferred that it be at a value whichwill cause the specific volume of the carbon dioxide to range from 0.8to 10.0 cubic feet per pound mol at reservoir temperature. The injectedcarbon dioxide is driven through the formation into contact with the oilwhich is to be displaced. The carbon dioxide is highly soluble inreservoir fluids and is generally much more soluble in oil than inwater. Due to its solubility in oil, when the carbon dioxide contactsthe reservoir oil a portion of it goes into solution with the reservoiroil, resulting in a viscosity reduction of the oil. In addition to theviscosity reduction, there is a preferential extraction from the oil bythe carbon dioxide of light intermediate hydrocarbons containing from 2to 5 carbon atoms, thereby developing an intermediate-rich carbondioxide bank in the vicinity of the line of contact between thereservoir oil and the carbon dioxide. Depending upon the composition ofthe reservoir fluids, particularly as to amount of intermediates, andunder proper conditions of temperature and pressure, theintermediate-rich carbon dioxide bank may be completely miscible withthe reservoir oil. Further, there is a swelling of the reservoir oil byvirtue of the dissolving of the carbon dioxide in it. The carbon dioxideshould be injected in an amount which provides a transition zone offlowing fluid from the reservior oil to the displacing liquidhydrocarbon slug. Such a transition zone includes a portion next to thereservoir oil which is a carbon dioxide-reservoir oil mixture. Next isflowing carbon dioxide in phase equilibrium with any nonflowing oil inwhich carbon dioxide is dissolved, followed by a carbon dioxide-liquidhydrocarbon mixture adjacent to the pure liquid hydrocarbon slug. Theregion of flow of primarily carbon dioxide phase need be no more than atrace since the basic objective is to provide a smooth viscositytransition from the reservoir oil to the liquid hydrocarbon displacingmaterial. The approximate quantity of carbon dioxide required may bedetermined by known procedures in laboratory-conducted floods undersimulated reservoir conditions. The amount will, of course, be affectedby reservoir conditions of temperature and pressure, together with thereservoir fluids characteristics.

The second step of the invention involves introducing a liquefiedhydrocarbon material into the formation through the injection wellbehind the carbon dioxide of step 1. The term liquefied hydrocarbons asused herein is intended to include such condensable hydrocarbons asliquefied petroleum gas, propane, butane, and light naphthas which undernormal conditions of temperature and pressure exist as a liquid. Thecondensable hydrocarbons, of course, are introduced and maintained at apressure which will retain them in the liquefied state. The liquefiedhydrocarbons are driven into contact with the carbon dioxide with whichthey have a high mutual solubility. The solution of carbon dioxide andliquefied hydrocarbons thus formed provides a transition from the carbondioxide to the liquefied hydrocarbons as discussed above. The gradualblending from the reservoir oil through to the liquefied hydrocarbonslug minimizes the fingering effect normally found in connection withthe use of liquefied hydrocarbon slugs as a displacing material with thefingering being the result of decided differences in viscosity betweenthe reservoir oil and the lique fied hydrocarbon slug. Due to theimprovement of the sweep efliciency of the liquefied hydrocarbon slugeffected by the carbon dioxide injected in advance of it, along with theswelling of the reservoir oil by the carbon dioxide, the quantity ofliquefied hydrocarbons necessary to efiectively displace the reservoiroil is significantly reduced below normal requirements.

Subsequent to the injection of the liquefied hydrocarbons, there isinjected into the formation through the injection well a driving fluidwhich functions to displace the reservoir oil, the carbon dioxide, andthe liquefied hydrocarbons through the formation to a production well,through which they are driven to the surface. The driving fluidpreferably is a dry hydrocarbon gas, such as a separator gas, consistingin major part of methane with minor amounts of ethane and trace amountsof higherboiling hydrocarbons. The driving fluid may, however, be a fluegas or air. In some cases it may be desirable to drive the injectedliquefied hydrocarbons with an amphipathic liquid followed by water. Anamphipathic liquid is a material having a mutual solubility with waterand a hydrocarbon fluid, such as an alcohol of three or four carbonatoms and an aldehyde or a ketone. Injection of the driving fluid iscontinued to eflect displacement of the reservoir oil through theproduction well until either all of the oil has been displaced from theformation or until the economical limit of the ratio of the drivingfluid to reservoir oil has been reached.

While the previously described steps of the method of the invention havebeen discussed in the sense of their being applied through a singleinjection and a single production well, it is to be understood that suchmethod is applicable to all of the various known well patterns whichmight be employed, such as the 5-spot system of well location.

What is claimed is:

1. In a method of recovering oil from a subterranean reservoirpenetrated by at least one injection well and one nuqsluctipawe r s d..,,.b in spa d apart one from the other, the steps which comprise:

( q llgnfilbiqnid oxide into ,said reservoir..

through said injection Well, and driving said carbon dioxide throughsaid reservoir, into contact with said oil, said carbon dioxide beingintroduced into said reservoir in an amount to provide Within saidreservoir a first transition zone, said first transition zone includinga portion next to said oil which is a mixture of said carbon dioxide andsaid oil, and a carbon dioxide phase following said mixture of saidcarbon dioxide and said oil;

(b) introducing a liquefied hydrocarbon material into said reservoirthrough said injection well, and driving said liquefied hydrocarbonmaterial through said reservoir, behind and into contact with saidcarbon dioxide phase, said liquefied hydrocarbon material beingintroduced into said reservoir at a pressure which will maintain saidliquefied hydrocarbon ma terial in the liquefied state in said reservoirto provide within said reservoir next to said carbon dioxide phase asecond transition zone, said second transition zone including a portionwhich is a mixture of said carbon dioxide and said liquefied hydrocarbonmaterial, and a zone of said liquefied hydrocarbon material whereby saidfirst transition zone, said carbon dioxide phase, and said secondtransition zone provide a smooth viscosity transition from said oil tosaid zone of liquefied hydrocarbon material;

(c) introducing into said reservoir through said injection well adriidng fluid to displace said oil, said transition zones, said carbondioxide phase, and said liquefied hydrocarbon material through saidreservoir toward said production well; and

(d) producing said oil from said reservoir through said production well.

2. A method in accordance with claim 1 wherein said carbon dioxide isintroduced into said injection well in the liquefied state.

3. A method in accordance with claim 1 wherein said carbon dioxide isinjected at a pressure which will cause i the specific volume of saidcarbon dioxide to range from i 0.8 to 10.0 cubic feet per pound mol atreservoir tem-i perature. i

4. A method in accordance with claim 1 wherein said 5 driving fluid instep (c) is a dry hydrocarbon gas consisting in major part of methane.

5. A method in accordance with claim 1 wherein said driving fluid instep (c) is an amphipathic liquid followed by water.

References Cited by the Examiner UNITED STATES PATENTS 2,742,089 4/1956Morse et a1. 1669 6 Rzasa et a1 1669 Martin et a1 1669 Stone 1669 Westet a1 166-7 X JACOB L. NACKENOFF, Primary Examiner.

CHARLES E. OCONNELL, Examiner.

S. J. NOVOSAD, Assistant Examiner.

1. IN A METHOD OF RECOVERING OIL FROM A SUBTERRANEAN RESERVOIRPENETRATED BY AT LEAST ONE INJECTION WELL AND ONE PRODUCTION WELL, SAIDWELLS BEING SPACED APART ONE FROM THE OTHER, THE STEPS WHICH COMPRISE:(A) INTRODUCING CARBON DIOXIDE INTO SAID RESERVOIR THROUGH SAIDINJECTION WELL, AND DRIVING SAID CARBON DIOXIDE THROUGH SAID RESERVOIR,INTO CONTACT WITH SAID OIL, SAID CARBON DIOXIDE BEING INTRODUCED INTOSAID RESERVOIR IN AN AMOUNT TO PROVIDE WITHIN SAID RESERVOIR A FIRSTTRANSITION ZONE, SAID FIRST TRANSITION ZONE INCLUDING A PORTION NEXT TOSAID OIL WHICH IS A MIXTURE OF SAID CARBON DIOXIDE AND SAID OIL, AND ACARBON DIOXIDE PHASE FOLLOWING SAID MIXTURE OF SAID CARBON DIOXIDE ANDSAID OIL; (B) INTRODUUCING A LIQUUEFIED HYDROCARBON MATERIAL INTO SAIDRESERVOIR THROUGH SAID INJECTION WELL, AND DRIVING SAID LIQUEFIEDHYDROCARBON MATERIAL THROUGH SAID RESERVOIR, BEHIND AND INTO CONTACTWITH SAID CARBON DIOXIDE PHASE, SAID LIQUEFIED HYDROCARBON MATERIALBEING INTRODUCED INTO SAID RESERVOIR AT A PRESSURE WHICH WILL MAINTAINSAID LIQUEFIED HYDROCARBON MATERIAL IN THE LIQUEFIED STATE IN SAIDRESERVOIR TO PROVIDE WITHIN SAID RESERVOIR NEXT TO SAID CARBON DIOXIDEPHASE A SECOND TRANSITION ZONE, SAID SECOND TRANSITION ZONE INCLUDING APORTION WHICH IS A MIXTUURE OF SAID CARBON DIOXIDE AND SAID LIQUEFIEDHYDROCARBON MATERIAL, AND A ZONE OF SAID LIQUEFIED HYDROCARBON MATERIALWHEREBY SAID FIRST TRANSITION ZONE, SAID CARBON DIOXIDE PHASE, AND SAIDSECOND TRANSITION ZONE PROVIDE A SMOOTH VISCOSITY TRANSITION FROM SAIDOIL TO SAID ZONE OF LIQUEFIED HYDROCARBON MATERIAL; (C) INTRODUCING INTOSAID RESERVOIR THROUGH SAID INJECTION WELL A DRIVING FLUID TO DISPLACESAID OIL, SAID TRANSITION ZONES, SAID CARBON DIOXIDE PHASE, AND SAIDLIQUEFIED HYDROCARBON MATERIAL THROUUGH SAID RESERVOIR TOWARD SAIDPRODUCTION WELL; AND (D) PRODUUCING SAID OIL FROM SAID RESERVOIR THROUGHSAID PRODUCTION WELL.